bu

nited

Available online 21 October 2014

Keywords:Lean premixed combustionThermo-acoustic instabilityPassive control

ene an

LPM combustion system without and with PIM placed on the dump plane. Measurements of instanta-

ations

to vortex breakdown and the associated recirculation zone in a highReynolds number ow [3,4]. Another important feature of swirl sta-bilized ows is the formation of shear layers at the interface of uidstreams with different velocities [5]. The hydrodynamic instabilityof shear layers is known as the KelvinHelmholtz instability, and itis usually convectively unstable [5].

ic fatigue failureacoustic instabil-are in phasnergy is ad

the system faster than it is dampened. In this case, the oscamplitude will initially increase exponentially until it saat some limit cycle [10].

The mechanisms responsible for exciting thermo-acousticinstabilities are numerous and some are still unknown. Thermo-acoustic instabilities often involve interactions between severalphysical phenomena such as unsteady ame propagation leadingto unsteady ow velocities, acoustic wave propagation, and hydro-dynamic instabilities [11]. Vortex formation in combustion cham-bers is also one of the main sources of ame/acoustic coupling [12].

Corresponding author.E-mail addresses: jwm3320@bellsouth.net (J. Meadows), AAgrawal@eng.ua.edu

(A.K. Agrawal).

Combustion and Flame 162 (2015) 10631077

Contents lists availab

Combustion

evicombustion instabilities arising from vortex interactions with theame front [2]. For example, the precessing vortex core (PVC)develops when a central vortex core precesses around the axis ofsymmetry at a well-dened frequency, and it usually contributes

strain the operational limits or cause catastroph[5]. According to the Rayleighs criterion, thermo-ities develop if the heat release rate uctuationsthe pressure uctuations [9] since the acoustic ehttp://dx.doi.org/10.1016/j.combustame.2014.09.0280010-2180/ 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.e withded toillationturatescombustion has gained increased utility for power generating gasturbines and related applications. LPM combustion strategiesreduce the ame temperature to decrease the thermal NOx forma-tion. However, LPM combustion is more susceptible to thermo-acoustic instabilities [1]. Recirculation zones are commonly foundin LPM swirl-stabilized combustion, and are known as sources of

ations in a turbulent ow eld [6]. Putnam [7] and Strahle [8]developed analytical models and empirical data to correlate soundpressure level (SPL) with operating parameters such as airfuelratio, fuel type, reactants ow rate, and geometry in non-premixedcombustion systems. Under certain conditions, sound waves cancouple with the natural acoustic modes of the combustor and con-Porous inert mediaCombustion noiseCombustion dynamics

1. Introduction

Due to stringent emission regulneous and average ow elds, vorticity, and turbulent kinetic energy are presented to gain insight intothe ow structure. Results are analyzed using the proper orthogonal decomposition (POD) to identifythe effect of PIM on the coherent turbulent structures in the ow eld. The addition of PIM removesthe recirculation zones and the dominant coherent turbulent structures formed in the shear layers areconvected downstream of the reaction zone. Harmonic reconstruction of the ow eld is performed atthe frequency of thermo-acoustic instability to identify the relationship between the ow and acousticelds of the system. Without PIM, the vortical modes in the corner recirculation zone are shown to bethe driving source for the instability. PIM alters the ow eld from global instability to convective insta-bility, which effectively eliminates the feedback mechanism for thermo-acoustic instabilities in the pres-ent LPM swirl-stabilized combustion system.

2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

, lean premixed (LPM)

One of the most challenging aspects of LPM combustor design isthe consideration of combustion noise and thermo-acoustic insta-bilities. Combustion noise is generated by heat release rate uctu-Received in revised form 28 August 2014Accepted 30 September 2014

combustion systems, although the underlying mechanisms involved have not been understood. The pres-ent study utilizes time-resolved particle image velocimetry (PIV) to measure the turbulent ow eld in aTime-resolved PIV of lean premixed comporous inert media for acoustic control

Joseph Meadows, Ajay K. Agrawal Department of Mechanical Engineering, University of Alabama, Tuscaloosa, AL 35487, U

a r t i c l e i n f o

Article history:Received 29 April 2014

a b s t r a c t

In the past, we have docummitigate combustion nois

journal homepage: www.elsstion without and with

States

ted the efcacy of the ring-shaped porous inert media (PIM) to passivelyd thermo-acoustic instabilities in lean premixed (LPM) swirl-stabilized

le at ScienceDirect

and Flame

er .com/locate /combustflame

ionThe time delay between vortex formation and the instant of heatreleased by combustion in the vortex can provide the phase rela-tionship between the oscillating pressure eld and unsteady heatrelease rate to drive the instability [11]. As such, uctuating vorti-cal ow structures tend to excite pre-existent pressure oscillations,thereby increasing the likelihood of combustion instabilities [13].In general, these instabilities occur near frequencies associatedwith the combustors natural longitudinal, radial, azimuthal, orbulk modes [14]. The acoustic eld can also excite hydrodynamicow instabilities, which leads to large organized vortical structuresnear the ame front to further introduce uctuations in the heatrelease rate [15,16]. The coupling between acoustic and hydrody-namic modes can occur through non-linear mechanisms, and theacoustic and hydrodynamic frequencies do not have to match tocreate thermo-acoustic instabilities [17,18].

A number of active and passive control techniques have beenutilized to mitigate thermo-acoustic instabilities [1923]. Activemethods usually consist of actuation of the fuel or air delivery sys-tem in response to the ame behavior while passive methods seekto modify the combustor geometry, for example, by bafes, resona-tors, and acoustic liners [22]. When instability occurs at one dom-inant frequency, passive techniques such as Helmholtz resonators,matching upstream and downstream lengths, gradual diameterchanges, and locating an orice at the antinodes of the quarterwave have all proven to be moderately effective at reducingthermo-acoustic instabilities, however, their practical implemen-tation in actual combustor design is often difcult [20]. Hermannet al. [24] attached cylindrical extensions to the burner nozzleand inclined them by 10 degree with respect to ow axis to reducethe formation of coherent structures, and thus, displace the com-bustion zone downstream of its former position to increase thetime lag to passively control the thermo-acoustic instabilities ina 260 MW heavy duty gas turbine.

Agrawal and Vijaykant [25] developed a passive technique tomitigate combustion noise and thermo-acoustic instabilities inswirl-stabilized, LPM combustion systems. This passive techniqueinvolves placing an open-cell structure of porous inert material(PIM) at the dump plane of the combustor. PIM is a ceramic annularring alloyed with hafnium carbide/silicon carbide (HfC/SiC) layeredcoating to resist high temperature oxidation in combustion envi-ronments, and can withstand operating temperatures up to1800 C [25]. PIM could be designed to t a wide range of dimen-sions for conventional combustors using advanced additive manu-facturing techniques, and as such can potentially extend theoperational limits of current gas turbine engines without costlydesign modications. The porous material itself is characterizedby porosity (percentage of void volume) and pore density expressedin terms of pores per cm (ppcm). Both porosity and pore density ofthe insert affect the ow structure and pressure drop across the PIM[26]. Marbach and Agrawal [26] have shown that PIM can be used toextend the lean blow-off limit of LPM combustion. Since, a highporosity PIM is desired tominimize the pressure drop, this and sub-sequent studies have utilized PIM inserts with 85% porosity, i.e.,only 15% of the ow area of the insert is blocked to the ow.

Sequera and Agrawal [1] used simplied computational uiddynamics analysis to gain preliminary understanding of the oweld without and with porous inserts. The combustor was modeledusing axisymmetric geometry with swirling ow in conjunctionwith a turbulent premixed combustion model based on the workof Zimont [27]. The 2D axisymmetric model revealed that the por-ous insert eliminates the corner recirculation zone, strengthens theswirling ow and central recirculation zone, and directs somecombustion products through the porous insert. A series of exper-

1064 J. Meadows, A.K. Agrawal / Combustiments were performed to assess the efcacy of PIM at mitigatingcombustion dynamics in swirl-stabilized LPM combustion [1]. Itwas shown that a divergent PIM annulus with pore density of 18ppcm (45 ppi) provided the greatest reduction in the total SPLs.Pore density of less than 18 ppcm caused the ame to stabilizewithin the porous structure, which decreased the ability of thePIM to mitigate combustion noise and instabilities. Sequera andAgrawal [6] and Williams and Agrawal [28] demonstrated theeffectiveness of porous insert to reduce combustion noise and/orinstabilities for a range of operating conditions. Smith [29] andBorsuk et al. [30] conducted experiments using a diffuser shapeporous annular ring insert with pore density of 18 ppcm. Experi-ments were conducted by varying the ow operating conditionsas well as the axial location of the swirler upstream of the combus-tor dump plane. In these studies, thermo-acoustic instabilitieswere observed without PIM in the combustor. In all cases, thethermo-acoustic instabilities were mitigated in the presence ofPIM inserts with peak SPL reduction of up to 30 dB.

While the PIM inserts are effective in mitigating combustiondynamics, and the responsible mechanisms have been hypothe-sized and discussed in the previous literature, a detailed under-standing of the underlying mechanisms remains unknown.Meadows and Agrawal [31] determined the absorption coefcientof PIM as a function of temperature and frequency, and for leandirect injection (LDI) combustion found that only a small fractionof the acoustic energy mitigated by porous media could be contrib-uted to the acoustic absorption of the material. The study hypoth-esized that other mechanisms such as, alteration of the turbulentow eld, redistribution of the heat release rates and changes infuel atomization and evaporation processes in the presence ofPIM inserts help to mitigate combustion instabilities. Meadowsand Agrawal [32] utilized time-resolved PIV to investigate thenon-reacting swirl-stabilized ow eld without and with PIM foracoustic control. POD analysis of data showed that PIM results ina more uniform distribution of turbulent energy in the ow eld.The precessing vortex core and corner recirculation zone were alsoeliminated with PIM.

The present study seeks to quantify the effects of PIM on theturbulent structures in a LPM swirl-stabilized combustion system,and to identify the coherent uctuations in the ow eld at the fre-quency at which thermo-acoustic instabilities are observed. Theoverall goal is to identify and explain the mechanisms responsiblefor mitigating combustion noise and thermo-acoustic instabilitiesin LPM swirl-stabilized combustion with PIM. First, the static sta-bility and dynamic stability of the ame are discussed by analyzingame photographs and dynamic pressure data. Next, time-resolvedPIV technique is used to provide insight into instantaneous andtime-averaged ow elds. POD analysis is used to quantify theeffect of PIM on the dominant coherent turbulent structures, andharmonic reconstruction of the ow eld is performed at the insta-bility frequency to explore the coupling between the ow eld andthermo-acoustics of the system.

2. Experimental setup

Figure 1 shows the swirl-stabilized combustor setup orientedvertically. The experiment is operated at atmospheric conditions.After passing through a pressure regulator, dehumidier, andwater trap, the air ow is split into primary air and seeded air owlines. The air ow is seeded with approximately 2 lm diametertitanium dioxide particles produced by a custom designed solidparticle seeder located 2.0 m upstream of the test setup to allowfor proper mixing. The seeded air rst ows through a plenumlled with marbles to breakdown the large vortical structuresand to further homogenize the seed particles in the air ow. Then,the air ows through the mixing tube and enters the cylindrical

and Flame 162 (2015) 10631077combustor via a swirler in an annulus with outer diameter of4 cm and inner diameter of 2 cm. The swirler consists of six vanesat 28 to the horizontal plane to produce a swirl number of 1.5. The

tionPIM

Exit Plane

Quartz Combustor

Dump Plane

Swirler

Reactants

L = 30.5 cm

D = 8.0 cm

Mixing Tube

D = 4.8 cm

Fig. 1. Schematic of combustor setup.

J. Meadows, A.K. Agrawal / Combusquartz cylindrical enclosure is 30.5 cm long with 8.0 cm innerdiameter. PIM consists of two porous rings stacked on top of eachother, with inner and outer diameters of 48 mm and 78 mm,respectively. Each PIM ring has height of 25.4 mm and the poredensity of 18 pores per cm.

The primary air ow rate is controlled by a manual valve andmeasured using a laminar ow element (LFE) with reported cali-bration error of 5 liters per a minute (lpm). The total pressureand pressure drop in the LFE are measured with an absolute pres-sure transducer and a differential pressure transducer respectively.The measured air ow rate is corrected for temperature as speci-ed by the LFE manufacturer. The seeded air ow rate is controlledby the supply pressure and measured using a sonic nozzle (Flow-Maxx SN08-SA-062), with a 60/40 percent split between the owrates of primary air and seeded air. Methane fuel ow rate is mea-sured and controlled by mass ow controller with measurementuncertainty of 1.5 liters per a minute. The total air ow rate enter-ing the combustor was set to 3.12 g/s (150 SLPM). Methane entersthe mixing tube approximately 61.0 cm upstream of the dumpplane at a ow rate set to obtain an equivalence ratio of 0.79.

Sound measurements are acquired by Bruel & Kjaer Model 8149condenser microphone. The condenser microphone was placed atthe exit plane of the combustor, and 30.5 cm away from the com-bustor outer wall to prevent the overheating of the sensor. Micro-phone output is converted to pressure uctuations using thesensitivity (45.8 mV/Pa) of the condenser microphone with adynamic range of 16.5134 dB and an uncertainty of 0.1 dB. Thecondenser microphone is calibrated by a piston-phone generatingpure tone of 114 dB at 251.2 Hz. The sound pressure measure-ments are sampled at 40 kHz for one second resulting in a fre-quency resolution of 1 Hz. Sampled data are processed usingLabView embedded fast Fourier transform (FFT) function to obtainthe sound pressure spectra. The sound measurements were usedmainly to isolate the acoustic frequency and to quantify thedifference in the SPL without and with PIM. The pressure uctua-tions within the combustor are not measured in this study, butthey are expected to be much greater.

Swirl-stabilized combustor ow consists of several complexphenomena which all can contribute to the system bifurcating toan unstable operating regime. Experimental techniques such asParticle Image Velocimetry (PIV) have been utilized to improvethe understanding of these interactions. For example, Wicksall[3336] investigated the effects of fuel composition on the oweld of ames using PIV and OH Planar Laser Induced Fluorescence(OH PLIF) techniques and observed differences in the ow eld ofdifferent fuels. Experimental investigation of detailed turbulencecharacteristics has been difcult in the past, because of the limitedtemporal resolution of non-intrusive optical diagnostic capabili-ties. However, recent advancements in high-speed lasers and digi-tal imaging systems make it possible to simultaneously resolveturbulent ow structures with a wide range of length and timescales. Computational and numerical simulations/models such asLarge Eddy Simulations (LES) and thermo-acoustic modeling areactive research areas [3739], and time resolved PIV has provenas an effective technique to provide extensive experimental valida-tion data leading to increased fundamental understanding of thecomplex physical processes.

OConnor and Lieuwen analyzed the multidimensional distur-bance eld caused by transverse acoustic excitation of a swirlingannular nozzle ow and a premixed-swirl stabilized ame usingtime-resolved PIV at framing rate of 10 kHz [40]. They showed thatthe ow eld near the nozzle is superposition of acoustic and vor-tical disturbances, and that different disturbances are observed indifferent portions of the ow eld. OConnor and Lieuwen alsoinvestigated the vortex breakdown bubble in a transversely excitedswirl ow [15]. In both of these studies [15,40], the non-reactingand reacting ow elds were compared to delineate the effectsof combustion. The dominant coherent structures, in particularthe center recirculation zone and inner and outer shear layers,were observed in both non-reacting and reacting ow elds. Theame mainly affected the size and aspect ratio of the center recir-culation zone, the shear layer spreading angle, and the magnitudeof the velocity perturbations and vortical disturbances. Morerecently, Steinberg et al. [41] utilized stereoscopic PIV, OH PLIF,and OH chemiluminescence to investigate the vortex structureand their interactions with the ame region. The ow eld wasfound to contain either periodically shed toroidal vortices or helicalprecessing vortex cores that excited the thermo-acoustic instabili-ties. Caux-Brisebois et al. [42] used proper orthogonal decomposi-tion (POD) analysis to identify helical precessing vortex cores in aswirl-stabilized combustor experiencing thermo-acoustic instabil-ity. Terhaar et al. investigated the key parameters governing thePVC both experimentally and analytically, and found that the exci-tation or suppression of the instability is related to the backowvelocity and density gradient in the shear layer [43]. Stopperet al. quantied the effect of pressure on the ow eld usingtime-resolved PIV [44]. Temme et al. used pressurespatial corre-lations and phase averaged PIV measurements to identify an equiv-alence-ratio oscillation instability mechanism [45]. Theseadvanced laser diagnostic techniques have proven ideal for inves-tigating combustion dynamics and thermo-acoustic instabilities.

In this study, the velocity measurements in the ow eld areobtained using the time-resolved PIV technique. QuantronixHawk-Duo 532-120-M Nd:YAG laser with wavelength of 532 nmand 18 mJ/pulse at the 4.2 kHz repetition rate is used for the exper-iments. The time between the two laser pulses was set to 50 ls.Schematic of the PIV experimental setup and laser/camera timing

and Flame 162 (2015) 10631077 1065diagram are shown in Fig. 2. TSI divergent sheet optic, withf = 25 mm cylindrical lens, combined with a 500 mm sphericallens, was used to create 1 mm thick laser sheet. Photron SA5

tor

LasPu

ion and Flame 162 (2015) 10631077Synchronizer

Laser

Front View

Combus

= 1

Laser Pulse Delay

Computer

/

1066 J. Meadows, A.K. Agrawal / CombustFastcam camera with Sigma 105 mm focal length lens and1024 1024 pixel sensor was used at framing rate of 8.4 kHz toimage eld of view of 65 mm by 70 mm. A 532 nm bandpass opti-cal lter is used to lter the undesirable light. The spatial resolu-tion for the experiment is 98 lm per pixel, which corresponds tominimum velocity resolution of 1.96 m/s.

Velocity eld calculations were performed using Insight 4GData Acquisition, Analysis, and Display Software from TSI. Thevelocity was computed using a two pass Recursive Nyquist Grid.The initial interrogation window size of 64 64 pixels with 50%overlap grid spacing was used. The computed velocity vectors thenproceed through local median vector validation with a referencevector used as the median value of all vectors in the neighborhood.The results from the previous pass are used to optimize the spotoffset for the next pass and the interrogation window is reducedby one half the size of the previous pass. FFT is used to computethe correlation function, and the location of the correlation peakis determined by tting a Gaussian curve to the highest pixel andits four nearest neighbors. The measured data were rejected basedon three criterions: passing the median test as mentioned above,peak to noise ratio of 1.75, and six-sigma validation. On averageabout 13% of the vectors are rejected mainly because of theunwanted reections in the quartz cylinder, which were sub-tracted out during preprocessing. A recursive lling using the localmean was used to ll the holes in the vector eld. The lling pro-cedure sorts the holes by the number of valid neighbors found ini-tially. It rst lls the holes with the most valid neighbors since theyhave the best chance to be lled; it then lls the holes with the sec-ond most valid neighbors, in which the holes lled in the previouspass are also treated as valid neighbors. The processing techniqueprovided 1.568 mm spacing between vectors. Based on the mini-mum velocity resolution, the spacing between velocity vectorsand the viscosity of air at 1500 C, the minimum turbulent Reynolds

Fig. 2. Schematic and timing diagramLaser

Camera

90

Camera

Top View

Camera

er 1 lse

Laser 2 Pulsenumber is 10.5, which signies scale resolution of about an orderof magnitude greater than the Kolmogorov scale.

3. Results and discussion

3.1. Flame stability

This section discusses the static ame stability and acousticbehavior of the system. Figure 3(a) and (b) show photograph andschematic of LPM swirl-stabilized combustion. The ame stabilizesimmediately downstream of the dump plane, see Fig. 3(a). The cor-ner recirculation zone is formed between the combustor wall andthe outer shear layer, and it recirculates hot gas products to ignitethe incoming reactants. The ame in Fig. 3(a) has a light blue1

appearance, which is characteristic of a lean low NOx emissionsame. The swirl effect is seen visually in the photograph as the ameinclines from the dump plane/swirler exit toward the combustorwall. The schematic in Fig. 3(b) highlights the typical ow structuresin a swirl-stabilized LPM combustor [3]. Reactants entering the com-bustor from the annular swirler produce inner and outer shear lay-ers. The vortices shed from the outer shear layer are located in thecorner recirculation zone, and the vortices shed from the inner shearlayer reside in the central recirculation zone. The ow structuresformed from inner and outer shear layers have been observed exper-imentally using time-resolved PIV [15] and computationally usingLES [46].

A photograph and schematic of LPM combustion with PIM areshown in Fig. 3(c) and (d) respectively. The photograph inFig. 3(c) depicts amelets stabilizing on the downstream surfaceof the PIM. The bulk of the reactant ow passes through the

for the PIV experimental setup.

1 For interpretation of color in Fig. 3, the reader is referred to the web version ofthis article.

on

wn

tionCorner Recirculati

Zone

VortexBreakdo

Bubble

J. Meadows, A.K. Agrawal / Combusopening in the center, and the downstream portion of the ame inthe core region is observed in the photograph. The core amestabilizes partially upstream and partially downstream of the PIMsurface as illustrated in the schematic in Fig. 3(d). The ameletson the surface of the PIM help stabilize the core ame. The staticame stability is favorably affected by the amelets since any largescale turbulent uctuations would be eliminated as the reactantow penetrates into the PIM. A non-porous structure of the samegeometry was also tested; however the static stability of the amewas rather poor. With a non-porous structure, the ame wouldeither shift back and forth within the core region and downstreamof the solid structure indicating an imbalance of ow velocity andturbulent ame speed, or it would stabilize downstream of the solidstructure. Thus, the ability of the reactants to ow through the por-ous insert and formation of amelets on the downstream surface ofthe PIM are the essential features of the present concept.

Acoustic measurements are acquired as described in theexperimental setup, and the sound pressure levels (SPL) as functionof frequency are determined using Eq. (1), and the total SPL isdetermined from Eq. (2).

Reactants

(a)Reac

Reactants

(c)Rea

PIM

Fig. 3. Photographs and schematics of swirl-stabilCombustor Liner

Inner and Outer Shear

Layers

Swirler

and Flame 162 (2015) 10631077 1067SPLf 10 log10P2rmsf P2ref

!1

where Pref = 20 lPa

SPLtotal 10 log10Xni1

100:1SPLi !

2

Figure 4 shows the sound pressure spectra with a distinct peakat 531 Hz. The longitudinal natural frequency of a similar experi-mental setup has been determined by Meadows and Agrawal[31] using acoustic wave equation with pressure antinode andnode boundary conditions at the inlet and outlet of the combustorrespectively. The longitudinal modes of the combustor is a functionof the combustor length (L), gas constant (R), ratio of specic heats(c), and the temperature (T); f l n

cRT

p=4L where n is the mode

number. Assuming air as the working uid and temperature rangeof 10001400 K yields the natural frequency range of the rst lon-gitudinal mode between 520 Hz and 615 Hz. Thus, the observedpeak SPL at frequency of 531 Hz is most likely an excitation of

(b)tants

(d)ctants

Core Flame Region

Surface Flame

ized LPM combustion without and with PIM.

ionthe rst longitudinal mode. With PIM, SPL peak occurs essentiallyat the same frequency, but the P2rms value decreases by 74% com-pared to the case without PIM. Note that the SPL peak withoutand with PIM occurs at the same frequency which indicates thatthe PIM has a negligible effect on the natural frequency of the rstlongitudinal mode of the combustor. The total SPL without andwith PIM is 92.0 dB and 87.6 dB, which corresponds to reductionin SPL of 4.4 dB by the PIM.

The addition of PIM in LPM combustion alters the fundamentalstatic ame stability mechanism and signicantly decreases theSPL at the frequency of the thermo-acoustic instability. WithoutPIM, the corner and center recirculation zones ignite the incomingreactants, and with PIM, the amelets on the downstream annularsurface of the PIM ignite and help stabilize the reactants owingthrough the core region. The corner recirculation zone is a knownsource of ame-acoustic coupling and PIM effectively eliminatesthis region of the ow eld.

Frequency (Hz)

Pressu

reVa

rianc

e

0 200 400 600 800 10000

0.005

0.01

0.015

0.02

0.025

0.03

Without PIM (Total SPL = 92.0 dB)With PIM (Total SPL = 87.6 dB)

Fig. 4. Pressure variance spectrum.

1068 J. Meadows, A.K. Agrawal / Combust3.2. Flow eld

According to the Rayleigh criterion [9], if the pressure uctua-tions are in phase with the heat release rate uctuations,thermo-acoustic instability will occur. Thus, the region where heatrelease rate uctuations occur is the dominant source of thermo-acoustic instabilities, and this region is analyzed without and withPIM in this study. Without PIM, the ow eld is measured imme-diately downstream of the dump plane, and the ame stabilizesin this region. With PIM, the ow eld is measured immediatelydownstream of the PIM where the ame stabilizes. The eld ofview is approximately 65 mm (transverse) by 70 mm (axial). Axiallocation, z = 0 mm corresponds to the combustor dump plane, andz = 50 mm corresponds to the exit plane of the PIM. The transverselocation, r = 40 mm corresponds to the combustor wall; however,velocity measurements were not acquired within 5 mm of thecombustion wall because of the distortion of the PIV image causedby the curvature of the quartz cylinder. A total of 1000 instanta-neous vector elds are obtained at a frequency of 4.2 kHz. The dataare processed using an ensemble average, and all data analyses areperformed using MATLAB. The out of plane vorticity is approxi-mated using forward differencing. The turbulent kinetic energy(TKE) is determined by multiplying 0.5 the sum of the varianceof the velocity components. TKE does not distinguish betweenthe periodic and turbulent uctuations rather it is a superpositionof the two. The SPL spectra revealed a small peak at the instabilityfrequency, and thus is expected that the periodic uctuations havea small impact on the magnitude of the TKE for the present testconditions.

Figure 5(a)(c) show the instantaneous velocity vector eldsuperimposed on the axial velocity contour plots without PIM.The succession of images from top to bottom corresponds to timestep of 6.0 ms. The inner and outer diameter of the swirler is20 mm and 40 mm respectively. Thus, no axial ow is observedin the central region between r = 10 mm at z = 0 mm. Also, thereis no axial ow at the dump plane in the corner region beyondthe swirlers outer diameter. The reactant ow entering the com-bustor accelerates as it tilts toward the combustor wall. A largerecirculation zone is present in the central region, which effectivelydecreases the ow area downstream of the dump plane. InFig. 5(a)(c), the central recirculation zone and the shear layersappear wrinkled indicating turbulent nature of the ow eld.

Figure 5(d)(f) shows instantaneous ow eld of LPM swirl-stabilized combustion with PIM. Similar to the case without PIM,the time step between the ow elds is 6 ms. The ow eld showshigher axial velocity in the center of the combustor indicating thatthe majority of the ow passes through the central void, and only asmall portion of the ow enters the annular PIM. Combustion takesplace in the core region both upstream and downstream of the PIMsurface. Connement of the ow area by the PIM and heat releasein the core region cause signicant increase in the axial velocity.The thermo-acoustic oscillations of the rst longitudinal mode willgenerate pressure waves traveling in the positive and negativeaxial direction. The pressure waves cause velocity oscillations, evi-dent in Fig. 5 where shear layers downstream of the PIM appearwrinkled, and the axial velocity contours reveal regions of bothhigh and low axial velocities.

The time-averaged axial velocity contourwithout PIM in Fig. 6(a)shows the jet ow entering the combustor. The regions of highvelocity gradients on either side of the jet ow are the inner andouter shear layers. Figure 6(a) reveals a large central recirculationzone and a relatively small corner recirculation zone. The ow eldappears symmetric with well-dened shear layers. Without PIM,the maximum average axial velocity in the eld of view is approxi-mately 6.5 m/s. With PIM, Fig. 6(b), the core ow at much highervelocity is observed at the exit of the PIM (z = 50 mm). The maxi-mum average axial velocity is about 11.5 m/s, which represents anincrease of over 75% compared to the case without PIM. The outershear layer between the core ow and PIM inside diameter isobserved at the PIM exit, while the inner shear layer is formed far-ther downstream. Figure 6(b) clearly demonstrates a decrease inthe shear layer spreading angle with the addition of PIM.

Vorticity represents the local spinning of the uid at a particularlocation. Average vorticity contours without and with PIM areshown in Fig. 6(c) and (d) respectively. Figure 6(c) shows thatwithout PIM, the vorticity is generated in the outer shear layeradjacent to the corner recirculation zone, which is a known sourceof thermo-acoustic instabilities [2]. The inner shear layer adjacentto the central recirculation zone also generates vorticity. With PIM,Fig. 6(d), a signicant increase in vorticity is observed in both innerand outer shear layers. However, the vorticity generated in theseshear layers is convected downstream because of the high axialvelocity of the core ow. Figure 6(e) and (f) show the TurbulentKinetic Energy (TKE) contours without and with PIM respectively.Without PIM, Fig. 6(e) shows that the TKE is highest in the centralrecirculation zone and the inner shear layer. With PIM, Fig. 6(f),TKE in the core region is much higher because of the interactionsbetween inner and outer shear layers downstream of the PIM. In

and Flame 162 (2015) 10631077the corner region downstream of the PIM, the TKE is relativelynegligible indicating that the surface stabilized amelets couldbe laminar.

tionYmm

40

60

V m/s7.5

5

2.5

J. Meadows, A.K. Agrawal / CombusFigure 7 shows transverse proles of average axial velocity (left)and TKE (right) at several axial locations, y = 1 mm, 20 mm, and40 mm, where y represent the axial distance from the referenceplane, z = 0 without PIM and z = 50 mm with PIM. At y = 1 mm,where the shear layers are initially formed, the average axial veloc-ity proles for the two cases are fundamentally different. WithoutPIM, Fig. 7(a), the axial velocity prole shows peaks on either sideof the centerline representing inner and outer shear layers oneither side of the swirling jet ow. The corner recirculation zoneis indicated by the negative axial velocities for |r| > 30 mm. WithPIM, Fig. 7(a) reveals a nearly top-hat prole of high axial velocity

(a)

(c)

(b)

X mm-40 -20 0 20 400

20 0

X mm

Ymm

-40 -20 0 20 400

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and Flame 162 (2015) 10631077 1069in the core region, and an outer shear layer at the intersection ofthe core ow and inside diameter of the PIM. Further downstreamat y = 20 mm, negative axial velocity is observed in the centerregion without PIM, Fig. 7(b). With PIM, the axial velocity is posi-tive at all transverse locations, which indicates that the bulk owin this region is owing downstream. The steep velocity gradientin the center signies the formation of an inner shear layer in thisregion. For y = 40 mm, without PIM, Fig. 7(c), the average axialvelocity is slightly negative at |r| = 10 mm signifying the end ofthe center recirculation zone located upstream. TKE representsthe velocity uctuations, and the highest velocity perturbations

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1070 J. Meadows, A.K. Agrawal / Combustare observed in the shear layer for all transverse proles, exceptimmediately downstream of the PIM in Fig. 7(d). In general TKEin the center region is much greater with PIM than that withoutPIM.

Typical ow features in swirl-stabilized combustion systemsincluding the inner and outer shear layers, and the central and cor-ner recirculation zones have been reported in several experimental[15,40,41,4347] and computational [3,38] studies. The axialvelocity proles in this study clearly demonstrate that the PIMeliminates the corner recirculation zone and the central recircula-tion zone is moved farther downstream and away from the ame.The central recirculation zone present without PIM is replaced by a

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and Flame 162 (2015) 10631077core ow of high axial velocity with PIM. The axial velocity uctu-ations in the shear layers are higher with PIM. However, thesevelocity perturbations are convected downstream because of thehigh, and positive, axial velocity of the bulk ow. Thus, the pertur-bations do not interact with the incoming ow, and the possibilityfor velocity perturbation feedback to the upstream is eliminated.Furthermore, the corner recirculation zone present without PIMis changed to an annular region of amelets with PIM. These am-elets help stabilize the ame in the core region, and convect, in thedownstream direction, any disturbances generated at the outershear layer between the core ow and inside diameter of the PIMto prevent upstream feedback of the ow disturbances.

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tion1240 mm Downstream of Reference Plane

J. Meadows, A.K. Agrawal / Combus3.3. POD analysis

Instantaneous and average ow elds offer insight into the tur-bulent structures present; however a method to quantify the

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and Flame 162 (2015) 10631077 1071energy contributions of different coherent structures to the oweld will provide a systematic approach to compare thesestructures. Proper orthogonal decomposition (POD) is an efcientanalysis technique to capture the dominant components of an

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innite-dimension process with only nitely many, and often sur-prisingly few, modes [48]. POD analysis was introduced in thecontext of turbulence by Lumley [49]. POD analysis decomposesa series of velocity elds into a set of deterministic functions andtime coefcients. An understanding of the coherent structurespresent in the turbulent ow eld can be inferred from the PODmodes provided the user has an intuitive idea of the structureswhich are present in the ow eld. The time-resolved data can alsobe used to determine the frequency of modes. POD analysis can beperformed using two methods: the direct method [50] or the snap-shot method [51]. Both methods lead to the same solution; how-ever, the direct method requires more computational time andmemory. In this study, the snapshot method based on Chen et al.[52] is used to compute the POD modes, and for each POD mode,fast Fourier transform (FFT) on the time coefcients is performed.POD analysis is performed on the uctuating velocity eldobtained by subtracting the average eld from the instantaneous

1072 J. Meadows, A.K. Agrawal / Combustioneld. The velocity eld can be reconstructed by summing the aver-age ow eld, hui, and a linear combination of all orthogonalmodes (Eigen functions), /ni weighted by the time coefcient, an,as shown in Eq. (3).

u^i hui XMn1

an/ni 3

where M is the number of instantaneous snapshots.In this study, 1000 instantaneous snapshots are used to decom-

pose the ow eld into spatially-dependent POD modes andtime-dependent coefcients. The modes represent the turbulentstructures in the ow eld, and each mode has an associated Eigenvalue that quanties the energy contribution of the mode. Theenergy contribution of each mode and the cumulative energy con-tribution of modes 110 are shown in Fig. 8. Without PIM, mode 1accounts for 22% of the turbulent energy. About 61% of the total tur-bulent energy is present inmodes 110.With PIM,mode 1 accountsfor 12.5% of the total turbulent energy, and 56.8% of the turbulentenergy is present in modes 110. A signicant reduction in cumula-tive contribution is observed in the rst two modes which contrib-ute to 32.8 and 22.8% of total turbulent energy without and withPIM respectively. Thus, PIM helps to distribute the turbulent energyacross a larger number of different turbulent structures.

Eq. (3) shows that every mode is present at each instance intime, and is weighted by a time coefcient. Thus, the FFT on thetime coefcients for a particular mode can yield the frequency con-tent of the mode. Figures 9 and 10 show POD modes 13 and the

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Fig. 8. Energy contribution of POD modes 110.associated spectra without and with PIM respectively. WithoutPIM, mode 1 in Fig. 9 clearly shows a large turbulent structure inthe central recirculation zone rotating about the axis of symmetry,which is characteristic of the precessing vortex core (PVC). Therotation of the PVC is difcult to observe in a stationary mode,but the frequency spectra of mode 1 reveal a well-dened fre-quency of 78 Hz. The PVC represented by mode 1 contributes to22% of the turbulent energy in the ow eld as shown in Fig. 8.The PVC is commonly observed in swirl stabilized ow and hasbeen veried both experimentally and computationally [3,4,4144]. Detailed analysis of heat release rate uctuations is necessaryto quantify the impact of the PVC on the thermo-acoustic behaviorof the system, and it is beyond the scope of the present work. Inthis study, pressure measurements taken outside the combustordid not reveal any peak at 78 Hz. In Fig. 9, modes 2 and 3 representturbulent structures mainly in the central recirculation zone, butunlike mode 1, the frequency of occurrence is not well dened.Note that modes 2 and 3 only contribute to respectively 10.0 and7.5% of total turbulent energy.

With PIM, modes 13 in Fig. 10 show that the coherent turbu-lent structures are conned to the inner shear layer formed down-stream of the PIM, and a well-dened frequency of occurrence isnot observed. Figure 8 shows, with PIM, 12.0, 10.0, and 5.0% ofthe total turbulent energy is contributed by modes 1, 2, and 3respectively. The PIM changes the ow eld such that the PVC isno longer present. When compared to the case without PIM, PIMbreaks up the turbulent structures such that the highest energymodes (i.e., modes 13) contain less of the total turbulent energy.

Recall from the ow eld data in Figs. 57, PIM alters the oweld such that the corner recirculation zone is eliminated. More-over, with PIM, the vortical structures formed in the shear layersare convected out of the ow. Accordingly, the turbulent structuresrepresented by modes 13 in Fig. 10 will be convected out of theow domain; thus preventing the possible feedback mechanismfor thermo-acoustic instability. The POD analysis provides amethod to quantify the energy contribution of turbulent struc-tures; however, it does not reveal the relationship between thevelocity eld and acoustic characteristics of the system. Time-resolved data allow for spectral analysis of the velocity eld, whichis utilized in the next section to quantitatively show the feedbackmechanisms responsible for thermo-acoustic instabilities.

3.4. Harmonic reconstruction

Time-resolved data can be used to identify the coherent uctu-ations at a particular frequency using the harmonic reconstructiontechnique outlined by OConnor and Lieuwen [15,40]. Harmonicreconstruction of the velocity eld is performed at the frequencyof instability. A FFT at every point in the ow eld is performedon both components of the velocity eld including the averageeld. The magnitude, A^~x, and the phase, u~x, of the FFT are usedto harmonically reconstruct the ow eld at the frequency ofthermo-acoustic instability, see Eq. (4).

u^~x; t Re A^~xeixtu~xh i

4

The term,xt, represents different phases in the harmonic wave.The analysis is a quantitative tool to identify the uctuatingcomponent of the velocity eld in the entire domain at a particularfrequency, and is used to visualize the global instability and theconvectively unstable ow eld without and with PIM respec-tively. By denition, a global instability or absolute unstable sys-tem occurs if a perturbation travels both upstream and

and Flame 162 (2015) 10631077downstream to affect the entire ow, and a convectively unstablesystem occurs if a perturbation introduced by a given ow is con-vected downstream by the mean ow [53].

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J. Meadows, A.K. Agrawal / Combustion and Flame 162 (2015) 10631077 1073

ion60 20000

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1074 J. Meadows, A.K. Agrawal / CombustThe harmonically reconstructed velocity and vorticity uctua-tions for finst = 531 Hz without and with PIM are shown in Figs. 11and 12, respectively. The vorticity quantities are derived from thevelocity eld reconstructions in Eq. (4). Without PIM, Fig. 11, themajority of the velocity vectors with large magnitude and associ-ated vorticity are located in the corner recirculation zone and outershear layers. The ow eld is oscillating at 531 Hz, and the differ-ent phases correspond to different times during one oscillationperiod. The excitation of the rst longitudinal mode is observed,

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and Flame 162 (2015) 10631077and forward and backward waves travel in the longitudinal (axial)direction [54]. The plane wave incident on the dump plane bound-ary can excite vorticity because of the transfer of energy from theacoustic to vortical modes [5], which can further excite vorticity inthe corner recirculation zone. The propagation of the disturbancesin both the downstream and upstream direction is indicative of anabsolutely unstable system. The coupling of the corner recircula-tion zone with the natural frequency associated with the rstlongitudinal mode creates the global ow instability, which is

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J. Meadows, A.K. Agrawal / Combusthe primary driving mechanism for thermo-acoustic instabilitywithout PIM.

With PIM, Fig. 12, the oscillating velocity eld is conned to theinner shear layer downstream of the PIM. The vorticity generatedin this region is convected downstream by the high velocity coreow, which can be visualized by examining the contours inFig. 12 in sequential order relative to phase angle. The high regionof vorticity at z = 75 mm and r = 5 mm moves approximately2.5 mm downstream as phase angle changes from 60 to 120degrees. The time between the two phase angles is 0.3 ms; thus

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and Flame 162 (2015) 10631077 1075the vortical structure is convected downstream at 8.33 m/s. Theaverage ow eld in the same region in Fig. 6(b) is approximately8 m/s; which matches with the vortex convection velocity deter-mined from Fig. 12. Thus, the perturbations introduced in the oweld are convected downstream by the mean ow and the systemis convectively unstable.

The harmonic reconstruction of the ow eld with PIM demon-strates coupling of thermo-acoustic instability with the vorticesshed in the inner shear layer downstream of the PIM, but these vor-tices are convected downstream by the core ow. PIM decreases the

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orticity uctuations for finst = 531 Hz with PIM.

prominent in the inner shear layer, which is also the region ofhigh axial velocity.

instabilities using subharmonic secondary fuel injection, J. Propul. Power 15

AIAA, Paper 20031009 (2003).[24] J. Hermann, A. Orthmann, S. Hoffmann, P. Berenbrink, Combination of active

ion4. The ow eld is harmonically reconstructed at finst = 531 Hz,and without PIM the velocity perturbations in the corner recir-culation zone couple with the rst longitudinal acoustic mode,and a global ow instability is observed. With PIM, the couplingof the velocity eld with thermo-acoustics is observed in theinner shear layer, but the vorticity generated in this region isconvected downstream by the high velocity of the core ow.

Without PIM, the corner recirculation zone is the primarysource for driving the thermo-acoustic instability in this study.Elimination of the corner recirculation zone by PIM effectivelydriving force for the instability since the system changes from a glo-bal instability without PIM to a convective instability with PIM.Lieuwen [5] discusses the difference between convectively andglobally unstable systems. A convective instability propagates onlyin the ow direction while a global instability can initiate at onelocation and then propagate in all directions of the ow eld. Previ-ous work by Meadows and Agrawal has shown that at finst the PIMwould absorb approximately 30% of the acoustic energy [31]. Thus,in terms of the Rayleigh criterion, with PIM, thermo-acoustic insta-bility is mitigated by simultaneously increasing the dampeningforce and decreasing the driving force in the system.

4. Conclusions

The ow eld for a LPM swirl-stabilized combustor without andwith porous inert media is analyzed. Thermo-acoustic instability isobserved at the frequency of the rst longitudinal mode of thecombustor, finst = 531 Hz, and a signicant decrease in SPL at thisfrequency and in total SPL is observed with the addition of PIMto the system. PIM has negligible effect on the frequency of the rstlongitudinal acoustic mode of the combustor. The combustor oweld is analyzed using time-resolved PIV, and the following conclu-sions explain the effects of PIM:

1. The static ame stability method without PIM is the recircula-tion of hot gas products to ignite the incoming reactants. WithPIM, the static ame stability mechanism is altered, and smallamelets with low velocity perturbations stabilize on thedownstream surface of the annular PIM. Flamelets ignite thereactants passing through the center and the ame stabilizesin the central void partially below and partially above thedownstream surface of the PIM.

2. Without PIM the vorticity is generated in the inner and outershear layers and corner recirculation zone, and the velocity per-turbations occur mainly in the center and corner recirculationzones. With PIM the vorticity is generated mainly in the innershear layer, and the shear layer spreading angle is reduced sig-nicantly. With PIM, the TKE in the central zone of the combus-tor is much higher, the corner recirculation zone is eliminated,and velocity perturbations are greatest in the inner shear layerlocated downstream of the PIM.

3. Without PIM, the rst POD mode of turbulent structures has anenergy contribution of 22%. A PVC rotating at a frequency of78 Hz is associated with mode one, and modes 2 and 3 repre-sent turbulent structures in the central recirculation zone. WithPIM, the rst mode has much smaller energy contribution ofonly 12.5%, and the PVC is eliminated. Without PIM, energy-containing turbulent structures are located mainly in the recir-culation zones. With PIM, the turbulent structures are most

1076 J. Meadows, A.K. Agrawal / Combustremoves the feedback mechanism for the instability. PIM alsoabsorbs a fraction (about 30%) of the acoustic energy. In terms ofthe Rayleigh criterion, thermo-acoustic instability is mitigatedinstability control and passive measures to prevent combustion instabilities ina 260 mW heavy duty gas turbine, 2001, DTIC ADP011146.

[25] A.K. Agrawal, S. Vijaykant, Passive Noise Attenuation System, U.S. Patent No.8,109,362, 2012.

[26] T.L. Marbach, A.K. Agrawal, Experimental study of surface and interiorcombustion using composite porous inert media, J. Eng. Gas Turbines Power(1999) 584590.[21] J.Y. Lee, E. Lubarsky, B.T. Zinn, Slow active control of combustion instabilities

by modication of liquid fuel spray properties, Proc. Combust. Inst. 30 (2005)17571764.

[22] S.M. Candel, Combustion instabilities coupled by pressure waves and theiractive control, Proc. Combust. Inst. 24 (1992) 12771296.

[23] A. Coker, Y. Neumeier, T. Lieuwen, B.T. Zinn, S. Menon, Studies of activeinstability control effectiveness in a high pressure, liquid fueled combustor,because PIM decreases the driving force and increases the acousticdamping of the system. Overall, with PIM, the present combustordesign changes from a globally unstable system to a convectivelyunstable system.

Acknowledgments

Joseph Meadows was supported by the Department of Educa-tion Graduate Assistance in Areas of National Needs (GAANN) Fel-lowship program. This research was supported in part by NASAGrant NNX13AN14A.

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J. Meadows, A.K. Agrawal / Combustion and Flame 162 (2015) 10631077 1077

Time-resolved PIV of lean premixed combustion without and with porous inert media for acoustic control1 Introduction2 Experimental setup3 Results and discussion3.1 Flame stability3.2 Flow field3.3 POD analysis3.4 Harmonic reconstruction

4 ConclusionsAcknowledgmentsReferences